1. Field of the Invention
This invention employs optical interferometry to measure the motion of a seismic mass in response to earth movements to yield a direct digital output signal proportional to particle velocity.
2. Discussion of the Prior Art
Conventional seismic transducers such as geophones measure particle velocity of the earth, the earth motion being due to ground unrest or to an acoustic disturbance. Typical commercial geophones have an active element consisting of a spring-suspended bobbin whereon is wound a coil of fine wire. The coil is suspended in a magnetic field. Relative movement between the coil and the magnetic field creates an analog voltage in the coil that is proportional to particle velocity. The analog output signals are transmitted to a signal utilization device over suitable conductors which, in commercial operations, may be several kilometers long.
During transmission, the analog signals become distorted due to such environmental problems as conductor losses, electrical leakage to ground, stray capacitance, and external interference from power lines and atmospheric electrical transients. The distortion is particularly distressing at low signal levels of a few microvolts per centimeter per second.
Many of the above problems may be alleviated by quantizing, at the geophone, the analog signals as digital data words. One convenient means for providing a digital output signal is to employ an optical sensor. In such a sensor, a moving mass is provided with a reflector. A beam of coherent light is reflected from the moving mass and is mixed with a directly-transmitted coherent light beam to create a pattern of interference fringes. A count of the number of fringes that occur within a unit time provides a digital number proportional to the velocity of the moving mass in terms of the wavelength of the coherent light beam.
Transducers that employ optical interferometry for measuring displacements of the velocity of motion of a moving mass are known. U.S. Pat. Nos. 3,693,400; 3,429,184 and 3,435,656 are examples. I have found that those devices are primarily designed for laboratory use under idealized conditions. They are unsuitable for field use. Typically the transducers are capable of measuring the displacement of the mass but not the direction of the displacement. The transducers count only the number of integral interference fringes that are observed. Fractional fringes are ignored. Hence the measurement resolution of the transducers is impaired.